The steam pressure in the boiler will be greater than atmospheric, so the steam temperature will be above 100C. A typical ready-built boiler boiler for steam powered models (sold with a pressure test certificate!) would have a maximum working pressure of 4 or 5 bar, which would give steam temperatures around 150C.
Many common plastics will start to soften at temperatures even below 100C, and there is no obvious reason why a computer fan should be designed to work at very high temperatures, so using computer fan parts is probably not going to work.
Temperature ratings of some common plastics are here: https://omnexus.specialchem.com/polymer-properties/properties/max-continuous-service-temperature. There are not many options for temperatures over 100C.
Live steam is not stuff you should play with unless you understand what you are doing. It is too easy to get third degree burns if things go wrong! A compressed air powered turbine might be a safer project, with no high temperature issues to worry about.